The Fifth Generation (5G) wireless network has the potential to radically change the communication infrastructure and business models across a broad range of fronts. Among the many technological advances in 5G networks is a brand new radio interface unconstrained by previous designs. This interface may employ massive MIMO and beam-forming technologies in a single antenna array that can greatly increase spectral efficiency with spatial diversity while supporting multi-users with increased signal to interference noise ratio (SINR).
Due to various advantages such as large beam-forming gain and spatial multiplexing gain, and high spatial resolution, massive MIMO communications have attracted considerable interest for future deployment in next generation of wide area wireless communication systems. In a massive MIMO system, a transmitter often has a very large number (i.e. from about 10 to over 100) of transmit antenna elements. To achieve high data transmission rate, one way is to transmit multiple data streams simultaneously through multiple antennas to a user equipment (UE). For example in FIG. 1, four separate data streams, d1(t), d2(t), d3(t), and d4(t), one for each antenna element, are transmitted to a UE at the same time achieving four times the d1(t) data rate. Another way to achieve high data transmission rate is to use beam-forming where a single data stream is split into multiple copies with phase shifting. Each of the phase-shifted copies is transmitted from each antenna, forming a beam signal, to the UE.
In a MU-MIMO system, the use of beam-forming increases SINR and capacity for each user. However, with the use of beam-forming, antenna calibration is required because the beam-forming algorithm relies on the precise knowledge of the phase and amplitude of the radio frequency (RF) signal. Yet, each RF chain, which may comprise a number of integrated circuits (RFICs), power amplifiers, and ADC/DACs, has various phase and power amplifier performance that are affected by temperature variations, and the undesirable effects of microstrip line and mechanical tolerances. Absent any calibration, the phase difference between the RF chains can be as high as +/−20 degree. This is unacceptable as beam-forming can tolerate a maximum of only +/−2 degree phase difference. Thus, antenna calibration is needed to provide the amplitude and phase correction in the RF feeds to the antennas.
Existing solutions include non-adaptive calibration, which involves taking measurements of the beam signal from the antenna array using an external network analyzer and adjusting the amplitude and phase for correction in the RF feeds; however, this solution greatly complicates the system's operation infrastructure with ad hoc additional expensive equipment and procedures.
One adaptive calibration solution is to use a separate calibration circuitry (implemented with i.e. DSPs or FPGAs) and separate calibration transceiver for each RF chain. In this solution, each RF chain has a directional coupler attached at the port directly before the antenna's feeding network and the coupler is connected to a separate receiver network having the calibration circuitry with a feedback path to the RF chain. A pilot signal is sent through all RF chains and measurements are taken at the couplers as input to the receiver networks for determining the amplitude and phase correction. This solution is accurate, but expensive with many additional components, such as combiners and power splitters in the calibration circuitries, added to the RF chains.
Another adaptive calibration solution is to utilize the mutual coupling properties between the antennas to compute the phase differences among them. However, because the return RF chains are not the same as the forward RF chains due to the presence of active devices, this solution is relatively less accurate, though less expensive than the aforesaid solutions.
In addition to the problems presented by antenna calibration, present beam-forming techniques also suffer the problem of excessive power consumption; for instance, in digital beam-forming (DBF), the RF chains, which must include numerous RFICs, power amplifiers, and ADC/DACs, consume large amount of power; and in analog beam-forming (ABF), the variable phase-shifters in the RF chains are very lossy.